1. Field of the Invention
The present invention relates generally to automatic focus control units and methods for automatically controlling a focal point by positionally adjusting a testing sample and an object lens. In particular, the invention relates to an automatic focus control unit and method, and an electronic device capable of performing focusing depending on an object lens when a plurality of object lenses are switched for use.
2. Description of the Related Art
In the field of optical equipment such as optical microscopes, depth meters, etc., auto focus devices have been used as devices for automatically focusing on a testing sample to be checked.
Examples of the automatic focus devices include one in which a laser beam is directed to a testing sample, a laser beam reflected from the testing sample is detected as the reflected laser beam, the positional relationship with the testing sample is determined based on the reflected laser beam for automatic focusing. This automatic focus device is provided with an optical detector such as a photo diode, which detects the reflected laser beam.
If the automatic focus device as described above is used in an optical microscope, a laser beam is emitted from a light source and directed to a testing sample via an object lens. The laser beam directed via the object lens becomes the reflected laser beam on the testing sample, which is directed to a photo detector via the object lens again.
The reflected laser beam forms on the photo detector a spot corresponding to the distance between the testing sample and the object lens. In the optical microscope mounted with the automatic focus device thereon, the object lens is shifted so that the reflected laser beam may form the spot at a predetermined position on the photo detector. In this way, the optical microscope performs focusing.
Known examples of focal point detecting methods used for microscopes include a knife-edge method, a differential spot-size method, an astigmatism method, a lateral shift method, and a Foucault method. Among them, a technique using the knife-edge method is generally used in the art.
The knife-edge method is a focal point detecting technique which uses a knife-edge mirror and a dual partitioning light-receiving element to perform automatic focusing. For example, as shown in FIG. 12, a laser beam emitted from a semiconductor diode 102 is directed to a testing sample 103 via a collimator lens 106 and via an object lens 101 while the knife-edge mirror 105 shields about half of the optical flux. The laser beam reflected from the testing sample 103 is reflected by the mirror surface of the knife-edge mirror 105 toward the dual partitioning light-receiving element 104. The dual portioning light-receiving element 104 is previously positioned so that the same amount of the reflected laser beam may be directed to two light-receiving sections constituting the dual partitioning light-receiving element 104 while the object lens 101 is focused on the front surface of the testing sample 103.
If the testing sample 103 is out of focus, then the laser beam reflected from the testing sample 103 is offset from an incident position of the dual partitioning light-receiving element 104. This causes a difference between outputs from the two light-receiving elements constituting the dual partitioning light-receiving elements 104. Accordingly, the knife-edge method achieves focusing on the testing sample 103 by shifting the testing sample 103 or the object lens 101 in the optical-axial direction to a position where the outputs from the two light-receiving elements become equal to each other.
Incidentally, in recent years, LCD panels, electronic devices, etc. have been formed with minute circuit patterns and substrates have increasingly been large-sized. Its manufacturing step and checking step have increasingly desired to specify and focus on a test object area with speed and accuracy. To meet the necessity, a parts test under a microscope has been proposed as below. A plurality of object lenses are provided which are different in magnification from each other. A low-power object lens is used when an alignment mark for a large-sized testing sample is detected, whereas a high-power object lens is used when a desired area of the testing sample is checked. That is to say, the object lenses are switched for use depending on the positions or states of a check portion.
In a focal point detecting method using the knife-edge method, however, if the object lens 101 is switched to cause the misalignment of optical axis or color aberration, then the incident position of the dual partitioning light-receiving element 104 will deviate as shown in FIG. 13. This needs such a device as to add a corrective value to the output from the dual partitioning light-receiving element 104 for each of the object lenses 101.
A focal point control method is proposed as a technique for correcting color aberration. In the focal point control method, a focusing error can be corrected even when a color aberration correcting lens is shifted in the optical-axial direction and an object lens having a different color aberration characteristic is used for replacement. This method may need a device for shifting the color aberration correcting lens without the misalignment of optical axis, which may desire high-accurate assembly. In addition, the increased optical parts will enlarge the configuration thereof as well as increase the cost of focal point detecting equipment. Thus, it is difficult to mount the focal point detecting equipment on the traditional, standard microscope.
See Japanese Patent Laid-open Nos. Hei 10-161195 and Hei 11-249027.